1) Field of the Invention
The invention relates to permanent magnet electrical machines. More specifically, the invention relates to an apparatus for reducing the cogging torque in a rotary permanent magnet electrical machine.
2) Description of the Prior Art
The rotary permanent magnet (PM) electrical machines are known to suffer from cogging torque. The cogging torque is an undesirable component of the torque of a PM machine and is caused by the interaction between the rotor permanent magnets and the stator poles of the machine. It causes undesirable vibrations of the machine and it is particularly prominent on start up of the machine.
Several methods have been proposed for reducing the cogging torque in PM machine. Those methods include:                uniform or variable air gap thickness;        phase shifting of the poles;        skewing of the stator poles or of the rotor magnets; and        use of fractional poles.        
Most of the proposed methods used for reducing the cogging torque also reduce the efficiency of the machine.
One proposed method for reducing the cogging torque of a PM machine is the mechanical coupling of two machines, each machine having a stator with electromagnets and a rotor with permanent magnets. The two machines are phase shifted such that the cogging torques of the two machines cancel out. In such a dual machine, most of the critical components are doubled and the cost of the machine is substantially increased.